Laser-ablated ruthenium atoms undergo reaction with acetylene during condensation in excess neon and argon matrices to form a metallacycle complex, insertion into the C H bond, and rearrangement to the vinylidene complex. The subject molecules were identified by (C2H2)-C-13 and C2D2, isotopic substitutions and density functional theory (DFT) frequency calculations. The HRuCCH molecule is described by Ru-H, CH, and CC stretching modes and CCH deformation modes. A very strong CC double bond stretching, weak CH stretching, and CCH deformation frequencies were observed for the Ru=C=CH2 complex. The metallacycle Ru-eta(2)-(C2H2) is characterized through CC double bond stretching, CH stretching and CCH deformation modes. The reaction mechanism for formation of the Ru=C=CH2 complex was investigated by B3LYP internal reaction coordinate calculations, and the hydrido-alkyny complex is the rate-determining step. The delocalized three-center-four-electron pi bond using the Ru 4d(xz) electron pair contributes to the C-C pi* orbital and provides stabilization energy (Delta E-(2), second-order perturbation) for the vinylidene Ru=C=CH2 complex.